The term hybrid vehicle refers to motor vehicles where at least two drive units are combined, relying on different energy sources for providing power to drive the vehicle. In a particularly preferred configuration, the properties of an internal combustion engine which produces kinetic energy by burning gasoline or diesel fuel, complements the properties of an electric machine which converts electric energy into motion energy. Modern hybrid vehicles are therefore predominantly equipped with a combination of an internal combustion engine and one or several electric machines. Two different hybrid concepts have been proposed. In the so-called serial or sequential hybrid concept, the vehicle is driven exclusively by the electric machine, while the internal combustion engine uses a separate generator to generate the electric current to charge an energy store that powers the E-machine, or alternatively powers the E-machine directly. More recently, however, parallel hybrid concepts are preferred at least in passenger vehicle applications, wherein the vehicle can be powered by both the internal combustion engine and by the E-machine.
The electric machines employed in these parallel concepts can be selectively operated in motor mode or in generator mode. For example, the E-machine operating in motor mode is typically added at operating points with higher vehicle loads in order to support the internal combustion engine. The E-machine can also operate as a starter motor for the internal combustion engine. Conversely, when the vehicle is powered by the internal combustion engine, the E-machine is operated predominately as a generator, with the electric power generated by the E-machine being used, for example, to charge the energy store and/or to supply a vehicle electrical system. With a split-power hybrid concept employing more than one E-machine, an E-machine operating in generator mode can also be used to supply power to another E-machine. In addition, at least a portion of the braking power is typically supplied by the E-machine operating in generator mode (recuperation), whereby a portion of the dissipated mechanical energy is converted into electric energy. Advantageously, with hybrid concepts, the E-machines generally operate with better efficiency than conventional claw pole generators.
Asynchronous machines (ASM) or permanent-excited synchronous machines (PSM) are examples of currently employed E-machines. With these types of machines, the efficiency disadvantageously decreases significantly at very low torque demand, i.e., when little electric power is supplied. Consequently, at a relatively low electric load, a disproportionately large amount of mechanical energy and therefore fuel must be supplied to the internal combustion engine to enable the E-machine to generate the required electric power.